REPRODUCTION 199 



conjugation. According to Wichterman (1936), conjugation in 

 Nyctotherus cordiformis (Fig. 85) takes place only among those 

 which live in the tadpoles undergoing metamorphosis (f-j). The 

 conjugants are said to be much smaller than the ordinary tropho- 

 zoites, because of the preconjugation fission (d-e). The micronuclear 

 divisions are similar to those that have been described for Para- 

 mecium caudatum and finally two pronuclei are formed in each con- 

 jugant. Exchange and fusion of pronuclei follow. In each exconjug- 

 ant, the synkaryon divides once to form the micronucleus and the 

 macronuclear anlage (k-l) which develops into the "spireme ball" 

 and finally into the macronucleus (m-o). 



A sexual process which is somewhat intermediate between the 

 sexual fusion and conjugation, is noted in several instances. Ac- 

 cording to Maupas' (1888) classical work on Vorticella nebulifera, the 

 ordinary vegetative form divides twice, forming four small indi- 

 viduals, which become detached from one another and swim about 

 independently. Presently each becomes attached to one side of a 

 stalked individual. In it, the micronucleus divides three times and 

 produces eight nuclei, of which seven degenerate; and the remaining 

 nucleus divides once more. In the stalked form the micronucleus di- 

 vides twice, forming four nuclei, of which three degenerate, and the 

 other dividing into two. During these changes the two conjugants 

 fuse completely. The wandering nucleus of the smaller conjugant 

 unites with the stationary nucleus of the larger conjugant, the other 

 two pronuclei degenerating. The synkaryon divides several times 

 to form a number of nuclei, from some of which macronuclei are 

 differentiated and exconjugant undergoes multiplication. In Vorti- 

 cella microstoma (Fig. 86), Finley (1943) notes that a vegetative indi- 

 vidual undergoes unequal division except the micronucleus which 

 divides equally (a), and forms a large stalked macroconjugant and a 

 small free microconjugant (b). The conjugation which requires 18- 

 24 hours for completion, begins when a microconjugant attaches it- 

 self to the lower third of a macroconjugant. The protoplasm of the 

 microconjugant enters the macroconjugant (c). The micronucleus of 

 the microconjugant divides three times, the last one of which being 

 reductional (d, e), while that of the macroconjugant divides twice 

 (one mitotic and one meiotic). Fusion of one of each produces a 

 synkaryon (/) which divides three times. One of the division products 

 becomes a micronucleus and the other seven macronuclear anlagen 

 (g, h) which are distributed among the progeny (i,j). 



Another example of this type has been observed in Metopus es 



